When using a ruler, a straight edge or scribe guide to aid in cutting a straight line in paper, fabric or other material, a sharp instrument is used. The sharp instrument may be a knife with a disposable or replaceable blade, a sharpened rolling disk attached to a handle or similar cutting device. The ruler is usually held in place by placing one hand on top of the ruler to position the ruler such that it is aligned with the line of the cut and firmly pressing down on the flat surface of the ruler. The cut is performed by drawing the cutting device along an edge of the ruler along a length thereof using the other hand. To ensure that the cut follows the straight line of the edge of the ruler, the blade of the cutting device may be positioned so that it contacts the edge of the ruler, and is held against the ruler by applying a transverse force to the cutting device while also performing the cutting operation. Rulers often have a tapered edge so that the edge portion contacting the cutting device is thinner than the remainder of the transverse cross section, so as to assist in accurately positioning a drawing or cutting implement with respect to the desired line when using the ruler.
The transverse force applied to the cutting device may cause the blade or sharp edge of the cutting device to ride up onto the tapered edge of the ruler and further towards the center of the transverse section, where the first hand is positioned. This may result in cutting injuries to the fingers of the user.